Behavioural responses to season and adult sex ratio in the fantail darter Etheostoma flabellare

Social behaviours such as competition for mates, which can indicate the strength and direction of sexual selection, may change either directly or indirectly in response to the operational sex ratio (OSR). Field studies indicate flexibility in social behaviours in response to a changing OSR, in particular as the OSR changes across the breeding season. Remaining unclear, however, is the degree to which these behavioural changes are a direct response to an unpredictably varying OSR, or are seasonal, developing in response to seasonal cues. In this experiment, we collected cohorts of male and female fantail darters at regular intervals over the course of two full breeding seasons and placed them in artificial streams in high, moderate and low adult sex ratios (ASRs). We observed streams for 5-day periods, during which we tallied aggressive and sexually solicitous behaviours. Both male and female intrasexual aggression responded to ASR manipulations as predicted, with males increasing and females tending to decrease intrasexual aggression in response to higher male numbers. Changes in intersexual aggression rates were not correlated with changes in intrasexual rates, indicating that these two classes of behaviour varied independently of one another. Male solicitations towards females increased at moderate ASRs, but decreased when the ratio of males to females exceeded the observed natural range. Contrary to predictions, female solicitations towards males increased with increasing male numbers. Male density and hence encounter rate was a confounding factor for interpreting male-directed (but not female-directed) behaviours in response to the ASR, and furthermore negatively affected overall reproductive success. We found no evidence that competitive or solicitous behaviour responds directly to seasonal cues in Etheostoma flabellare; rather, competition over mates and solicitation of mates appear to respond directly to changes in the ASR. [Copyright &y& Elsevier]